The central goals of the proposed investigation are to study the structure and function of a family of protein tyrosine phosphatases, termed STEP. STEP is specifically expressed within the CNS, and is found within neurons of the striatum, amygdala, nucleus accumbens, hippocampus, and cerebral cortex. The proteins consist of both cytosolic and membrane-associated isoforms. Its presence in the postsynaptic densities of neurons suggested that STEP was involved in signal transduction pathways, which we established over the last cycle of the grant. Over the past four years, we have discovered that STEP regulates the function of key signaling proteins, including the NMDA subtype of glutamate receptor and members of the mitogen-activated protein kinase and tyrosine kinase families. These findings have opened a new direction of research. We plan to continue characterizing the regulation and function of STEP through biochemical, molecular, immunohistochemical, and electrophysiological experiments. We will identify novel phosphorylation sites in STEP, identify the kinases and phosphatases that regulate these sites, and determine the functional significance of phosphorylation at these sites. Preliminary data suggests that other proteins associate with STEP, in addition to the three we have discovered to date. We will identify these STEP-interacting proteins and determine the functional significance of these interactions. However, the fact that STEP regulates both NMDA currents and the MAPK family suggests it is involved in LTP as well as aspects of learning and memory. We will test the new hypothesis through functional disruption of STEP. This will be accomplished in primary cell cultures and in vivo by using inactive TAT-STEP proteins as well as the STEP knockout mouse. These studies will clarify the physiological and functional significance for STEPregulated signaling in the brain and may provide information about neuropsychiatric disorders associated with alterations in learning and memory function.